In modern telecommunication systems, it is becoming ever more important to increase the density of data transmitted over any particular transmission line. As such it is advantageous to increase the number of the effective bandwidths useable by each transmission line. The advent of the Internet in the past few years has further accelerated the race for higher data transmission density.
When the transmission line is a fiber-optic cable, one method of expanding the effective data transmission capacity is to transmit a number of closely spaced optical frequencies on each cable. Such optical frequencies are also referred to as "carrier frequencies". By separately modulating and demodulating each such carrier frequency, the amount of information that may be carried on one fiber-optic cable can be substantially increased. This technique is called "Wavelength-Division Multiplexing" (WDM) and, when the spacing between the wavelengths gets very small, the technique is referred to as "Dense Wavelength-Division Multiplexing" (DWDM).
In a typical telecommunications application, laser diodes are used to provide optical signals which are transmitted through fiber optic cables. Presently, these signals are produced by a series of laser diodes whose output is a series of carrier wavelengths (frequencies) separated by a specified amount. For example, several diodes each producing signal at a different wavelength separated by 100 GHz (gigahertz) produce a composite signal which may be directed down a single optical fiber. These carrier frequencies are modulated and multiplexed to carry a multiplicity of signals on the same optical fiber. At the receiving end of the fiber, the carrier frequencies are demultiplexed and demodulated.
Multiplexing and demultiplexing of the carrier signals may be accomplished by various means, including optical gratings or coated optical interference filters. However, the use of such optical gratings and interference filters present certain problems which have not yet been overcome in the industry. Conventional air-spaced etalon filters have also been used to separate carrier signals as well as provide frequency standards used to monitor the lasers generating the carrier signals. However, current methods of producing precision etalons is a highly specialized "craft" more in the nature of an "art". Furthermore, such etalons are typically manufactured in small quantities of only a few units at a time. Currently, there is no known method of mass producing etalons having sufficient optical quality for this application. As a result, etalons of this type are not suitable for applications requiring large numbers of etalons (e.g. telecommunications applications).